An inertial navigation system (“INS”) and a global positioning system (“GPS”) generate position information on an individual. The inertial navigation system and the global positioning system generate complementary position information. The position information generated by the global positioning system may be used to correct the position information generated by the inertial navigation system for some measurements. The position information generated by the inertial navigation system may be used during reacquisition of satellites by the global positioning system. A filter (e.g., a Kalman filter) is used to weigh and combine the position information received from the inertial navigation system and the global positioning system. The accuracy of the position information on the individual is dependent on the reliability and availability of the inertial navigation system and the global positioning system. If either the inertial navigation system or the global positioning system become unreliable and/or unavailable, then the position information determined by the filter becomes less accurate. If both the inertial navigation system and the global positioning system become unreliable and/or unavailable, then no position information is generated.
As one shortcoming, the inertial navigation system has a position error (e.g., drift) that builds up over time. As the elapsed time of operation increases, the position information generated by the inertial navigation system becomes less accurate. There are times when the elapsed time of operation is long compared to the drift performance of the inertial navigation system. During such times, the position information determined by the filter becomes less accurate.
As another shortcoming, there are times when the global positioning system is unavailable due to jamming or interference. During such times, the position information determined by the filter becomes less accurate.
As yet another shortcoming, upon initialization and/or re-initialization, the inertial navigation system requires a starting and/or restarting position to begin generating the position information of the individual. Without the external input of the starting and/or restarting position, the inertial navigation system is unable to begin navigation. Also, upon initialization and/or re-initialization, a delay exists between the start of initialization and/or re-initialization and when the global positioning system is able to begin navigation. The delay is reduced if upon initialization and/or re-initialization the starting and/or restarting position of the global positioning system is available. There are times when an accurate starting and/or restarting position is unavailable.